Data communications between computers is accomplished in a variety of ways. So-called local area networks, as well as wide-area networks, are networks of computers that permit the exchange of data between computers by using either hard wired connections or two-way radio communications devices that permit data to be exchanged between machines.
It is anticipated that some local-area networks will soon become wireless by using some form of transmitted electromagnetic waves to exchange data between computers. So-called wireless local-area networks will avoid the costs associated with establishing, maintaining, and re-configuring hard-wired local area networks. It is anticipated that wireless local area networks will use radio signals instead of light or infrared signals because in part of the problems associated with reliable propagation of visible light waves or infrared waves in an office environment.
In the United States, the Federal Communications Commission (FCC), as of 1985, authorized the unlicensed use of the frequency bands between 902-928 MHz, 2.4-2.483 GHz., and 2.725-5.85 GHz. To qualify for unlicensed use, the FCC has imposed certain requirements that limit transmitter power and the time a user can broadcast on any one frequency in these bands, in order to prevent one user from monopolizing one or more channels. A wireless local area network that uses one or more of these bands would have to limit the output power of its radio transmitters and might have to be able to hop between frequencies.
To reliably transmit data on a wireless local area network, whereon the subscriber units are continuously hopping from one radio channel to another, requires the transmitters and receivers of such a network to maintain their synchronization to each other. As a transmitter hops from one channel to another, receivers of the transmitter's signal must be able to detect, demodulate, and synchronize with the signal from the transmitter.
Most embodiments of wireless, radio frequency (RF) LAN's that use a channel hopping methodology require the transmitter to send a synchronizing sequence, or preamble, of digital signals ahead of any data to permit a receiver to locate and synchronize itself to the transmitter. Since all digital sequence detectors used in a receiver require a finite amount of time to detect a particular digital sequence in a preamble as an indication of synchronization, reducing the amount of time that it takes for a sequence detector to achieve complete synchronization might offer significant benefits to an RF LAN that has its transmitters and receivers continuously hopping between channels. Reducing the time that it takes a receiver to lock up with a transmitter might allow more data and less preamble to be sent in a given time interval.
Those skilled in the art will recognize that virtually all useful digital data in a computer or other digital device is comprised of parallel digital data words or bytes. Sending these parallel digital quantities is most often accomplished by sending the data serially. Present day data communications using conventional modems exchange data between computers over telephone lines by sending data serially. Serial data is typically organized into binary digits, or bits, of course, but in addition, contiguous bits in the digital information typically comprises words, or bytes. In a serial bit stream, in a serial data communication network, including LAN's, there is a grouping of bits, often referred to as a frame, that is made up of several bits and perhaps several words or bytes. In most serial data networks, accurate reconstruction of the original digital information requires both a bit synchronization (which is the detection of individual bits in the serial information stream) and a frame synchronization (which is the detection of the start and stop points of a frame of digital information).
In an RF LAN, or any other serial data communications system, reducing the time it takes to achieve both bit and frame synchronization would improve the efficiency of the system. As the length of the preamble sent ahead of the data of interest is decreased, more of the data of interest can be sent in its place. Accordingly, any method or apparatus that reduces bit and frame synchronization time would be an improvement over the prior art.